This invention relates to methods and apparatus for milling windows in well casings or liners.
Wellbores drilled through the earth""s subsurface may be vertical, deviated or horizontal. Moreover, the wells may have one or more lateral branches that extend from a parent wellbore into the surrounding formation. After a wellbore has been drilled, it is typically lined with a casing and/or another liner. The casing extends from the well surface to some distance within the wellbore. Liners on the other hand may line other portions of the wellbore. The casing or liner is typically cemented in the wellbore.
In some cases, it may be desirable to change the trajectory of a wellbore after a casing or liner has been installed. Also, to form a multilateral well, one or more lateral branches are drilled and completed after a casing has been installed.
To change the trajectory of a well or to form a lateral branch from a cased or lined wellbore, a window is formed in the casing or liner to enable drilling of the surrounding formation. Generally, the casing is cut by one or more mills that are mounted on a mandrel at the bottom of a drill string. The mills may have abrasive elements made of sintered tungsten carbide brazed to their surface. When the drill string is lowered into the wellbore, it is deflected toward the casing by a deflection tool with a slanted surface, such as a whipstock. The whipstock may be set in the wellbore either during that run or a prior run. The whipstock is placed at a location in the well where the window will be formed.
Typically, as shown in FIG. 1, a milling assembly 10 includes a pilot mill 18 at the end of a mandrel 16 to provide an initial cut in the casing or liner 13. One or more spaced apart gauge mills or reaming mills 20, 22, 24 may follow the pilot mill 18. The peripheral surface of each mill has abrasive or cutting inserts (not shown) that are made of a hard material such as sintered tungsten carbide compounds. After the initial cut made by the pilot mill 18 in the casing or liner 13, the mills 20, 22, and 24 behind the pilot mill 18 enlarge the pilot window to form a full gauge window.
The mills 20, 22, 24 mounted on the mandrel 16 are able to ultimately form a continuous window in the casing or liner 13. However, because of the arrangement of spaced apart mills on a conventional milling tool, this window is first formed in discrete zones. As shown in FIG. 2, the cuts 26, 28, 30, and 32 formed by the mills 18, 20, 22, 24 at one point are discontinuous and will remain so until the milling process is near completion. That is, each mill 18, 20, 22, and 24 enlarges a discrete opening 26, 28, 30, and 32 in the casing 13 that lengthens and deepens over time. These openings are lengthened and widened until they eventually become one continuous full gauge window. This process may create large cuttings when the zones begin to overlap. The large debris may be difficult to remove from the well.
Moreover, milling operations may require different sized mandrels and mills to mill full gauge window. For example, a casing having a first size may require the use of a mandrel having a first diameter whereas a casing having a second size may require the use of a mandrel having a second larger diameter. Alternately, the same mandrel may be utilized in both casings; however, mills may need to be exchanged for differently sized casings.
Thus, a need for an improved milling apparatus and method continues to exist.
In general, according to one embodiment, a method of milling a window in a liner comprises arranging a plurality of milling elements substantially continuously along a rotatable mandrel and actuating the mandrel to cut a window through the liner. The window is cut substantially continuously using the milling elements to a desired size.
Other or alternative features will become apparent from the following description, from the drawings, and from the claims.